The present invention relates to venturi mixers used for delivering fuel and air mixtures to burners and pilots. More particularly, but not by way of limitation, the present invention relates to combustion assemblies comprising venturi mixers having burner tips, pilot tips, or other types of combustion tips attached thereto.
Premix burners and pilots have long been employed for producing and burning gaseous fuel and air mixtures in applications ranging from residential furnaces to large process heaters. Premix burners typically use venturi mixers to combine and mix the gaseous fuel and air components and to deliver the resulting fuel/air mixture to a combustion tip (e.g., a burner tip or a pilot tip). As used herein and in the claims, the term xe2x80x9cventuri mixerxe2x80x9d refers to and includes any venturi, eductor, or similar device wherein the ejection and/or flow of a gaseous fuel is used to draw air into the device and which includes a divergent discharge section from which the resultant mixture of gaseous fuel and air is delivered. Commonly, as fuel under pressure enters the venturi mixer, a low-pressure zone is created that acts to pull air into the mixer. For each cubic foot of fuel ejected into the venturi, between three and eight cubic feet of air may be educted into the venturi depending upon the efficiency of its design.
In a particularly preferred type of venturi mixer, fuel gas under pressure is ejected from an orifice toward the center of a convergent (typically bell-shaped) opening at the rearward (upstream) end of the venturi. A straight section or throat follows (i.e., is positioned forwardly/downstream of) the convergent inlet section. The throat connects, in turn, to a discharge section of the venturi having a divergent interior wall. A combustion tip is then typically welded or threadedly connected to the venturi at the exit end of the divergent discharge section. Fuel gas and air are mixed within the throat and the divergent section of the venturi to provide a uniform combustion mixture which is then delivered through the combustion tip. As used herein and in the claims, the term xe2x80x9ccombustion tipxe2x80x9d refers to a burner tip, a pilot tip, or any other type of combustion tip structure employed for delivering a fuel/air combustion mixture from a venturi mixer into a combustion area preferably downstream of (i.e., outside of) the tip.
Venturi mixers operate in accordance with Bernoulli""s Theorem. Because of the high velocity of the fuel gas jet delivered from the fuel gas ejection orifice, a negative pressure is created at the entrance of the venturi. This negative pressure zone continues into the straight section or throat of the venturi and draws air into the throat where the air begins to mix with the fuel gas. In the divergent section of the venturi located downstream of the throat, the gradual enlargement of the flow passage operates to convert the velocity pressure of the fuel gas and air mixture into static pressure. The resultant static pressure provides the motive pressure needed to expel the fuel/air mixture from the flow port(s) of the combustion tip.
Heretofore, premix burners have used methods of attaching the combustion tip to the venturi which have had little regard for minimizing pressure drop at the end of the divergent section. The attachment of the combustion tip often imposes a significant pressure drop due to the existence of a sudden contraction or sudden enlargement at the venturi/combustion tip interface. This pressure drop can significantly and undesirably reduce the degree of static pressure regain attained in the apparatus. Thus, it is an object of the present invention to significantly reduce or eliminate such pressure drop at the exit of the venturi divergent section as it transitions to the burner or pilot tip.
Depicted in FIG. 1 is a prior art combustion assembly comprising: a venturi mixer 10; a combustion tip 20; and a fuel gas supply pipe 12 containing a gaseous fuel 14 under pressure. Gas supply pipe 12 terminates in a fuel metering orifice 16. Gaseous fuel 14 is emitted from orifice 16 in a jet 18. Combustion air 20 is educted into the bell-shaped inlet 22 of mixer 10. The gaseous fuel jet 18 and combustion air 20 are mixed in the venturi throat 24 and in the venturi divergent section 26. As mentioned above, the purpose of the venturi divergent section 26 is to convert the velocity pressure of the fuel/air mixture exiting the throat 24 into static pressure. Burner or pilot tip 28 is secured at the exit end of the venturi divergent section 26 and includes outlet flow ports 30. The fuel/air mixture exits combustion tip ports 30 and is combusted in furnace space 32.
In prior art devices of the type depicted in FIG. 1, the combustion tip 28 is received in the exit end of the venturi divergent section 26 and is typically threadedly connected or welded thereto. In this prior art assembly, an abrupt contraction 34 of the type shown in FIG. 1 is commonly imposed upon fluid stream as it flows into combustion tip 28. The flow obstruction 34 creates a significant pressure drop at the divergent zone exit.
Another typical prior art combustion assembly is shown in FIG. 2. The prior art assembly of FIG. 2 is very similar to the prior art assembly of FIG. 1 except that, in the embodiment of FIG. 2, the discharge end of the divergent section of a venturi mixer 40 is received in a combustion tip 44. The burner tip or pilot tip 44 is commonly attached by threaded means or by welding. The combustion tip 44 includes one or more flow ports 46 through which a gaseous fuel/air mixture 50 exits the tip 44 to be consumed in the furnace space 48. A significant pressure drop occurs in this apparatus because of an abrupt expansion 52 at the interface of the divergent section 42 and the combustion tip 44.
The present invention provides a venturi mixer and a combustion assembly which alleviate the problems and satisfy the needs discussed above. In the inventive design, the divergent section of the venturi mixer extends into or otherwise interfaces with the interior of the combustion tip in a manner such that abrupt expansion or sudden contraction is either eliminated or at least greatly reduced.
In one aspect, the present invention provides an improvement for a combustion assembly. The combustion assembly includes a venturi mixer and a combustion tip attached to a discharge end section of the venturi mixer. The discharge end section of the venturi mixer has a divergent interior wall. The improvement comprises an extension of the discharge end section which projects the divergent interior wall into the combustion tip. The divergent interior wall preferably terminates at a sharp distal end of the extension which is positioned inside and adjacent to the interior wall of the combustion tip. In this improved assembly, the sharp distal end of the extension preferably touches the interior wall of the combustion tip.
In another aspect, the present invention provides an improvement for a combustion assembly including a venturi mixer and a combustion tip attached to a discharge end section of the venturi mixer wherein the discharge end section has a divergent interior wall and the improvement comprises: the combustion tip having an interior shoulder, the discharge end section of the venturi mixer having a forward end substantially abutting the interior shoulder; the divergent interior wall having an interior diameter at the forward end of the discharge end section of the venturi mixer; and the combustion tip having an interior diameter at the interior shoulder which is substantially equal to the interior diameter of the divergent interior wall at the forward end of the discharge end section of the venturi mixer.
The benefits derived from the present invention will be apparent to those skilled in the art. Because of the reduction in pressure drop at the venturi exit, the device is able to induce more combustion air. Thus, the premix gas burner tip or pilot tip is capable of operating over a wider air to fuel ratio range. Further, the flame propagation speed of the inventive assembly desirably increases as a result of the induction of more combustion air. In addition, with the increase in flame propagation speed comes a desirable increase in flame temperature as well as increased flame stability.
Another benefit of the present invention is the ability of the burner or pilot tip to operate at a higher internal pressure. The port(s) of the combustion tip can therefore be made smaller because of increased discharge pressure. Moreover, a higher internal pressure in the tip desirably provides greater ability to increase the tip exit velocity when needed to match the high flame propagation speeds encountered with certain fuel compositions.
Matching the tip exit port velocity to the flame propagation speed is of particular benefit for burners and pilots used in refinery applications. In an upset condition in a petroleum refinery, the fuel supplied to the burners in the process heaters typically contains a higher percentage of hydrogen. Hydrogen has a very high flame propagation speed. Thus, a low exit velocity at the burner tips or pilot tips can result in flame instability and flashback. However, the burner and pilot assemblies produced in accordance with the present invention can be designed to provide exit velocities exceeding the worst-case flame propagation speeds, thus resulting in greater stability and resistance to flashback.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following detailed description of the preferred embodiments.